Basement Insulation

Hello all - remodeling basement in 100+ year old Seattle house. Basement was 'finished' when we bought 15 years ago - though musty, and generally not well done - nothing really square or plumb. Now we're tearing into it to create a 'mancave' for the teenager, his friends, guitars, etc.

The issue is insulation. I've read extensively on the buildingscience.com website and am trying to hew to their recommendations as closely as possible.

A couple issues:
The wall construction is concrete wall at varying height - it gets lower with the grade, from about 6' at highest to about 3.5' at shortest. There is a 2x6 pony wall of similarly varying (inversely) height atop that.
When the house was built, 1x6 shiplap was attached to the inside of the 2x6 pony wall, and on the outside, metal lath directly on the 2x6 wall, with concrete stucco. This construction had degraded over the years and had allowed water in/trapped water in numerous places resulting in sill plate rot. I've removed the stucco all around, and replaced the compromised sill with new treated wood and a gasket, and replaced studs as needed. Outside is now clad in 1/2" plywood, building paper, and soon, cedar shingles. My question is how to insulate the pony wall.

I'm adhering 1.5" XPS to the concrete, to avoid condensation. (Previous 'finishing' was no insulation, and internal 2x4 wall up against the concrete wall - there was rot in places on the floor, and mold, though I think this was ground water - and I've regraded the outside and added a French drain, and injected urethane foam into some cracks).
I plan to frame internally with 2x4, and use unfaced fiberglass in the stud bays of the new internal framing.
I think I want to spray foam the band joist space to 1-2"; there will be additional fiberglass behind that - we're adding radiant tubing to the joist bays for the first floor, there will be insulation below this, and I was going to turn the batts up at the ends, behind the new spray foam.

What is unresolved is the 2x6 pony wall. In reading here and other places, I'm thinking a layer of foam (1"?) on the inside face of the 1/2" plywood exterior sheathing, followed by fiberglass, or just fiberglass? Should I insulate the pony wall, and not the internal framing? I'm thinking that the goal needs to be suitable total R value, though keeping the fiberglass from reaching dewpoint of interior air, and thus condensing, creating mold-growing potential.

It seems that with greater R value, and hence less heat flux/greater efficiency, the dew point moves more interior through the wall assembly. Is the goal keeping the dew point out of the insulation that will support mold growth, ie fiberglass?
Is there a role for a vapor barrier in the pony wall, to keep interior/moist air out?

The goal is to keep the WOOD above the dew point- condensation never occurs in the fiber insulation itself (strange, but true.) Where the wood IS below the dew point, it can be protected by a shot of closed cell foam, making the foam the condensing surface, of a material that doesn't wick or store moisture. But good results can be had by maximizing the drying capacity of the assembly in both directions, while severely restricting the AIR (not water vapor) flow from the interior.

On the foundation wall with known moisture issues it's better to use 2" of unfaced EPS rather than 1.5" of XPS, since it has about the same R-value but 3-4x the drying capacity. At a thickness 1.5" of XPS is less than 1.0 perm (0.6-.75 perms.depending on density) - semi-impermeable, slowing the drying of groundwater/rainwater in the foundation toward the interior, but it's not a disaster. With a sill gasket and pressure treated replacement sill the framing can handle much higher moisture content in the foundation itself, but watch for efflorescence & spalling on the exterior of the above-grade portion of the foundation. Be sure to put a sill gasket or something under the bottom plate of the interior studwall as a capillary break as well. With a 2x4 studwall and unfaced R11 or R13 batts, fully 1/3 of the center cavity R is on the exterior, and in a Seattle climate the temp at the fiber/foam interface will pretty much ALWAYS be above the dew point of the interior air. The whole-wall R of that assembly will be around R18.

The wire-lath on bare studs stucco was probably the larger moisture source though, since the stucco both absorbs dew/rain (does it still rain in Seattle? ), and is/was the condensing surface from interior moisture drives.

On the pony wall you'll get more benefit (both thermally and moisture-wise) by putting the 1" foam on the exterior side of the exterior plywood, since that puts a semi-permeable waterproof layer of R5 between all of the structural wood and the exterior, increasing it's average wintertime time (== drier), and protecting it from rain/dew drives. R5 is sufficient exterior R to allow you to insulate the 2x6 with air-permeable fiber and no interior vapor barrier (other than standard latex on gypsum) with virtually no risk of wintertime moisture accumulation. By avoiding strong vapor barriers it maximizes the drying capacity of the assembly, since it can dry toward the inteiror readily through the 2-5 perm paint, but also to the exterior through the ~1 perm 1" XPS. The condensing surface in the cavity will be the inside of the exterior sheathing, but it's average temp will be well above 40F (the dew point of 70F, 35% RH interior air), even in January, even if the basement's average temp is in the low-60s, assuming you're not actively heating the basement. (If you are heating the basement the exterior sheathing's temp will average a degree or two warmer.) With an inch foam on the exterior the whole-wall R of the assembly will be about R20.

If bumping it out an inch is going to interfere with your cedar shingle siding too much, use 1/4" purpose-made fan-fold XPS (~0.7 perms), or if you can 1/2" XPS (~2.2 perms). The R-value of the foam won't quite let you get away with simply paint as the interior-side vapor retarder on the pony wall- smart vapor retarders such as Certainteed Membrain would be best since it'll allow more drying than kraft-facers on batts, and will be above 2 perms most of the time for maximal drying capacity.

I'm not sure exactly what you're talking about with the "sheath / FG / poly / sheath / FG / interior gypsum/ latex paint" stackup. Is that to say the non-structural studwall holding the finish gypsum will overlap the interior of the pony wall? If yes insert a firestop about 6-8" above the top of the top of the concrete, and stop the 2x4 batts there, or bump up the exterior foam on the pony wall to 1.5" (at which point the R value of the ponywall + interior studwall will add up to something super-insulated.)

If you lay a bead of caulk on the framing as you put up the interior sheathing on the pony wall you can make it pretty air-tight without excessive use of spray foam. Sealing the seams with fiber-reinforced duct-mastic also helps. Similarly, making the inteior gypsum air tight with caulk as you go can limit the air-infiltration into the cavity. A square inch of air leak through a painted gypsum wall is worth a whole wall's worth of vapor diffusion. Most condensation problems in walls have far more to do with air infiltration than with vapor diffusion, especially in climates as mild as Seattle, where even the mean January OUTDOOR temp is only ~40F, with ample drying hours over the season.

Dana - thank you!
As you suspect, I cannot really accommodate a full inch on the exterior of the pony wall, though 1/4 might be do-able. (I've already got the building paper up - can it go on the exterior of this?)
What I meant by the layering I described was a description of a possible approach to the pony wall insulation, from the outside -> inward.
I was intending to run the non-structural 2x4 wall from floor to ceiling, so the cross sections would go like this (from outside ->in):

What about this:
-1/4" fan-fold on the exterior, under the cedar shingles.
-within the 2x6 cavity, 3" XPS, R15.
-within the 2x4 cavity, 3.5" FG, R13

Total R for the whole assembly would be about R=30 (including <1 R for the various plywood layers.
At 20' design temp, there is a 48' differential, more than haft the R value in the layers of XPS, such that the inner surface of the 2x6 cavity layer is about 46.6', or above the dew point.

I thought you had 1.5" of XPS between the foundation wall and 2x4 studwall, but that doesn't show up in your ponywall level stackup???

It literally doesn't matter if there is condensation at design temp even if it's on wood, since the duration of design temps is low, and the preponderance of the time it's above the dew point and drying. It's the AVERAGE temp over WEEKS that adds up to whether or not there's potential mold/rot issue related to condensation, not the 97.5th percentile or higher extremes. Using 25 year weather averages for the coldest 4-week period for Seattle there's very little risk of rot from condensation even without interior vapor retarders as long as it's very air-tight to the interior, but during the coldest 4-week period during any of the past 25 year (rather than the average) there is some risk, and a modest amount of vapor retardency between the exterior sheathing an interior side air is called for.

Also, don't confuse center-cavity R with whole-wall R- the thermal bridging of the ~R0.85/inch studs and plates robs the whole assembly of performance. With 3" of insulation in a 5.5" cavity, the thermal bridging of the studs is R2.5, but with a full cavity fill the thermal bridge would be ~R4.7. With your stackup the whole-wall R is closer to R20, not R30. (R10 for the 2x4 & gypsum, R8.5 for the 2x6 w/XPS due to the severe thermal bridge, ~ R1.5 for the plywood + shingles.)

An R19 batt installed as-designed in a 2x6 cavity only delivers R18(!) according to the manufacturers' own data. An R19 batt is the same type and amount of material as an R13, batt- it's MISERABLE STUFF, losing a large chunk of performance due to convection currents within the batt at the temperature extremes, but as long as you compress it to 4.5" (where it'll performs at ~R16) it's R-value is more stable over temp. If going full-depth in a 2x6 framing with batts, use high-density "cathedral ceiling" R21 batts. R22 batts are only slightly better than R19 batts, but only perform at R19 when installed per the manufacturers' instructions, but R21 batts perform to spec and lose far less R across the entire operating temperature range. (Crazy but true: An R21 batt has a higher installed-R than an R22 batt see: http://www.owenscorning.com/around/insulation/CompressionChart.xls )

On the pony wall, unless you air-seal the hell out of the cut'n'cobble 3" XPS it's not going to help any condensation issue, since air will convect around it through any possible route. And with only 3" of insulation rather than a full cavity fill the thermal short of 3" of stud is only about R2.5 which robs that expensive XPS of performance immensely, and leaves 2.5" of empty cavity to

For the money (and time) it would cost for 3" of XPS, you'd be better off doing a flash-foam of 1" closed cell spray on the interior side of the exterior sheathing at the same time that you do your foundation sill & band jois, then compress an R19 batt in there. The 1" of foam would give you R6, and an R19 batt compressed to 4.5" gives you another R16-17 for a total of R23 (center cavity). The thermal bridging of the 2x6 studs and top/bottom plates reduces that section of the stackup to about R15-16 for whole-wall R. Adding on the 2x4 section added on you'd be looking at ~R25, and the foam becomes a moisture-tolerant condensing surface, protecting the exterior plywood.

If you're not going to put any foam on the exterior (yes, it can go over the felt) extending the 1.5" of XPS so that it separates and thermally breaks the studs between the two studwalls would give you better thermal & and moisture performance, since that puts a vapor retarder (the XPS) in the middle of the R:

The shingles + 2 layers of ply add only about R1.5 to the stackup, and R21 batts in the 2x6 cavity gets cut down to ~R15 by the thermal bridging of the 2x6 framing, so you have R16.5 on the outside of the XPS. R13 batts in a 2x4 layer is only worth ~R10 after thermal bridging so you'd have R7.5+ R10 or R17.5 to the interior of the plywood, total R34 (whole-wall, not center cavity). The condensing surface (the inner face of the XPS) is less than a third of the way to the exterior, and always about the dew point. The 2x6 pony wall dries primarily to the exterior through the 2-5 perm felt, but has some capacity to dry to the interior thorough the XPS. You could skip the 2x4 fiberglass at the pony wall level if you wanted to and settle for ~R24 whole-wall, but it's probably easier & more reliable to install the 2x4 batts full-length.

Foundation wall level:

dirt or air/concrete/1.5" XPS/ batts in 2x4 cavity/ gypsum/ paint.

Whatever you do at the pony-wall level, don't leave empty space between the studs anywhere. Try to thermally break the wall with foam between the studs- even an inch of XPS will do a world of good performance-wise, and it's the "right" place for a vapor retarder. And bear in mind that at any layer in the stackup, air-retardency is far more important than vapor-retardency- caulk & spray foam is great, but a compressed R11 batt is WAY better than free-air between the framing.

Above grade or below, seal the seams of the XPS with duct-mastic to block convection currents.

To help me understand, I've drawn some pictures (see below). The first is just the wall as presently constructed (showing shingles which are not yet applied, though)
The second shows two possibilities as I understand your discussion.
It seems simpler to go with the one on the right - no exterior fanfold XPS, no spray foam in the cavity; with R21 FG in the 2x6 space and R13 FG in the 2x4 cavity, and 2" of rigid XPS applied to the interior face of the sheathing.

I had been thinking about spray foam in the 2x6 cavity, on the interior side of the exterior sheathing, but that seems both expensive and messy for a first-time DIYer like myself. I like the simplicity of gluing and sealing XPS boards to the plywood.

I was considering the spray foam (with the above trepidation) for the band joist space - or using foil-faced poly-Iso pieces cut to fit and sealed with canned foam.

Attached Files:

You've take the 1000 words and made the pretty picture, and methinks you've interpreted it right!

The one on the right is definitely fewer steps,an probably less expensive.

A few details to attend to for preventing air infiltration and/or convection losses & thermal bypasses:

When installing the interior plywood, put a bead of caulk on the framing at the edges as you nail it up to make it air tight.

Leave a 1/2-1" gap between the upper XPS and lower XPS at the top of the foundation, filling it with 1 part foam (or a small FrothPak kit). The XPS can be glued in place with foam-board construction adhesive to both the concrete and the plywood, tacked lightly in place the plywood with a few nails as you go. Try to keep them as co-planar as possible, since that affects the fit & function of the 2x4 studwall insulation.

Seal the top of the XPS to the plywood in your plumbing & electrical channel to prevent convection currents in the thin space between the XPS and the plywood using either foam board construction adhesive or spray foam.

Don't forget to use a sill gasket under the bottom plate of the 2x4 studwall between the plate and the slab. A bead of caulk under the plate also prevents air leakage under the plate into the wall.

If there's a big gap (more than 1/4") between the studs and the XPS, use unfaced R19 batts rather than R13 or R11 batts. Compressed to 3.5" an R19 batt will be ~R13, but it will have sufficient loft to not leave a convection path in the gap whereas an R13 batt wouldn't have a tight fit.

Use air-tight techniques on the interior gypsum board too (beads of caulk between the framing & gypsum as you put it up, foam seal around any electrical or plumbing penetrations, etc.)

The basement will probably end up being the tightest & warmest place in the house, eh? ;-)